Since the Second World War, two-wheel drive vehicles with a four-wheel drive option have become increasingly popular. These vehicles had shift on the fly, or 4.times.4 action at the operator's command. As time passed and the desires for fuel economy grew, it was desirable to not back-drive a front differential assembly, thereby reducing friction and increasing fuel economy. During that time, all or most four wheel drive vehicles had solid front axles or differential assemblies able to full-float the front wheels through the use of locking hub assemblies.
With the desire to give the operator more convenience, (not having to stop, get out, and lock hubs to provide 4.times.4 action) the industry designed a split-axle system, thereby back driving only the axles and axle gears. This system created a need to have all gears rotating in the same direction before engagement, but a time delay was required. These systems--including vacuum cable, electronic shift motors, solenoid, and thermogas operated--have a high failure rate and do not always operate instantly when needed. The systems further lack the ability to be controlled from inside the cab by the operator at his or her will, and the systems incorporate unnecessary vacuum or electronic sources.
An example of the more recent technology is shown and described in U.S. Pat. No. 4,699,235. As disclosed therein, most modern four-wheel drive vehicles utilize a transfer case that provides powerflow to the front and rear differentials. In two-wheel drive, the transfer case allows the powerflow to only the rear differential unit while isolating the front differential unit. Consequently, the transfer case allows for superior fuel economy and less gear wear when the vehicle is driven on improved roads where four-wheel drive is not required. In four-wheel drive, the transfer case provides power to both the rear and front differentials allowing for increased vehicle control.
Some four-wheel drive vehicles also contain a split-axle mechanism including a split-axle clutch. The split-axle clutch is positioned between the front right wheel's axle and the front differential axle (thus, the wheel's axle is "split" from the front differential axle). When engaged, the clutch provides lock-up between the front axle and the wheel axle allowing powerflow to both front wheels.
Connecting the powerflow from the transfer case to the front axle is accomplished by the front drive shaft and front differential unit. The front differential unit contains a series of planetary gears permitting the front wheels to be driven at different speeds while the vehicle is turning. In four-wheel drive the entire differential unit (and the planetary gears) conjunctively rotates with the front axle. Thus, through a series of gears (pinion and ring gear), the front differential unit accepts powerflow from the transfer case and delivers it to the front axle.
In two-wheel drive the transfer case and the split-axle clutch are disengaged. Additionally, the front differential unit remains motionless as a result of not being driven by the transfer case. Nonetheless, the forward motion of the front wheels backdrives the axles and front differential's planetary gears.
The backdriven axles rotate the planetary gears and the opposing axle gear in a direction opposite to the direction they rotate when the vehicle is in four-wheel drive or when the split-axle clutch is in a locked position. As a result, when the transfer case is activated to place the vehicle in four-wheel drive, a time delay is required before engaging the clutch to enable the planetary gears to slow down, stop, and begin proper rotation.
Backdriving the front differential unit also causes excessive wear on the axle and planetary gears. As a result, metal grindings created by the planetary gear's wear circulates through lubricant around the front axles, causing damage to other parts in the front axle.
To shift into four-wheel drive, the driver activates a switch inside the passenger compartment to cause the transfer case to switch power to the front axle. The clutch can not be activated, however, because the planetary gears are not rotating in the proper direction. As power increases from the transfer case, the planetary gears begin proper rotation.
Meanwhile, with the technology described in U.S. Pat. No. 4,699,235, a linear actuator is operated to engage the clutch. The linear actuator has an electric heater in a chamber filled with thermally expansible and contractible pressure transmitting fluid capable of undergoing a liquid to gas phase change upon heating. When the heater is activated, the gas expands and pushes a piston rod connected to the clutch assembly which engages the clutch. The heating process delays the action of the actuator for a sufficient time to allow the planetary gears to rotate in the proper direction.
One problem with this prior art approach is that the procedure to transfer from two-wheel to four-wheel drive may take several minutes or more, depending upon the ambient temperature surrounding the vehicle; the colder the weather, the more time needed to heat the actuator gas.
Another problem is wear on the front wheel drive differential. The metal grindings created by the wear of the axle and planetary gears circulates via the differential fluid throughout the entire front axle, possibly damaging other lubricated parts.
Yet another problem with the prior art approach is that when the vehicle is switched off, the gas cools in the linear actuator and the split-axle clutch disengages. When the vehicle is restarted, the process of heating the gas begins again. As before, several minutes may be required before the split-axle clutch engages and the vehicle is actually in four-wheel drive.
Accordingly, an object of the invention is to provide an apparatus that (when locked) instantaneously converts a two-wheel drive vehicle into four-wheel drive at the operator's command. Another object of the invention is to provide an apparatus that reduces the wear of the front wheel drive differential of a vehicle. Yet another object of the invention is to provide an apparatus that maintains a vehicle in four-wheel drive when the vehicle is turned off. Yet another object of the invention is to provide such a apparatus that is low cost and reliable. These and other objects of the invention will become more apparent from the following detailed description of a preferred embodiment.